Smithsonian at the Poles: Contributions to International Polar

338 SMITHSONIAN AT THE POLES / EISERT AND OFTEDAL
the vulnerability of populations to annual or long-term
changes in prey availability, as might occur due to changes
in sea ice or shifts in water currents.
MASS CHANGES DURING
WEDDELL SEAL LACTATION
Prior work on Weddell seals has focused primarily
on mass changes of mothers and their pups, under the assumption
that if mothers are fasting, there should be correspondence
between maternal mass loss, maternal milk
output, and pup mass gain, as is the case in other true seals
that fast throughout lactation. In these species, maternal
body mass and age are strong determinants of total milk
energy output and, consequently, of pup growth and weaning
mass (Iverson et al., 1993; Fedak et al., 1996; Arnbom
et al., 1997; Mellish et al., 1999b). By contrast, females
feed during a variable proportion of lactation in almost
half of extant phocid species (Bonner, 1984; Oftedal et al.,
1987a; Boness et al., 1994; Boness and Bowen, 1996;
Lydersen and Kovacs, 1999; Eisert, 2003). Bowen et al.
(2001a) found that the positive correlation of maternal
body mass with pup weaning mass was much weaker in
harbor seals than in species that fast during lactation, presumably
because supplementary feeding results in a partial
decoupling of maternal mass loss and milk transfer to
the pup. Similar patterns have been found in ice-breeding
FIGURE 1. Change in body mass of lactating Weddell seals from
early (2– 3 days postpartum) to late (35– 45 days postpartum) lactation
at Hutton Cliffs, Erebus Bay, McMurdo Sound. Data were
obtained from 24 females in 2006 and 2007. Average rate of mass
loss was 1.0% of initial mass per day.
grey seals H. grypus and harp seals P. groenlandica (Baker
et al., 1995; Lydersen and Kovacs, 1996, 1999).
Extant data for Weddell seals are more complex. Weddell
seal females certainly lose a large amount of body mass:
for example, females that we studied in 2006 and 2007 lost
40% of their two-day postpartum mass during about 40
days lactation (Figure 1). The daily mass loss of 1.0% of
initial mass is lower than values of 1.5%– 3.4% for fasting
and lactating females of the northern elephant seal, southern
elephant seal, land-breeding gray seal, and hooded
seal (Costa et al., 1986; Carlini et al., 1997; Mellish et al.,
1999a, 1999b), but Weddell seal lactation is so prolonged
that overall mass loss (42%) is equal to or greater than that
in the other species (14%– 39%). If mass loss is standardized
to a lactation length of 42 days, initial mass predicts
66% of the variation in mass loss, indicating that large females
lose more mass than small females (Figure 2). Is this
because large females expend more energy (on metabolism
and milk production) or because they feed less? Females
that lose more mass also support more mass gain by their
pups: pup mass gain was positively correlated to maternal
mass loss (Figure 3). Tedman and Green (1987) found a
similar strong positive correlation (r � 0.85, P � 0.001)
between maternal mass loss and pup mass gain, whereas
data from studies by Hill (1987) and Testa et al. (1989)
indicate a much weaker correlation between maternal mass
loss and pup mass gain (r � 0.16, P � 0.005, n � 35).
FIGURE 2. Relationship of maternal mass loss to initial maternal
mass of lactating Weddell seals. Initial mass was measured at two
to three days postpartum. Mass loss was normalized to 42 days and
compared to initial mass by Deming linear regression. Data are from
the same 24 females at Hutton Cliffs, Erebus Bay, McMurdo Sound,
as in Figure 1.